Some motorized vessels, e.g., a small open boat equipped with low-horsepower (e.g., 3 HP) outboard motor, is steered simply by manipulating the motor tiller. Such manipulation changes the angular position of the propeller and of a rudder-like piece adjacent to the propeller.
Motors of boats equipped with higher-horsepower outboard motors, the propeller-and-rudder drive of boats having inboard-outboard drives (often referred to as "I/O" drives) and the rudder of boats having inboard drives all exhibit higher tiller arm torque or high rudder torque (as the case may be), especially when the boat is turned. Cable-type mechanical systems have been used for steering such boats but the "steering force feedback" from the rudder to the hands of the operator is objectionable. And in a boat with two helms (two locations from which the boat can be operated), cable systems are difficult to install.
Hydraulic steering systems, in wide use, overcome the objectionable aspects of cable systems. Hydraulic systems include check valves preventing force feedback to the operator when the helm wheel is not being turned. And such hydraulic systems are quite easy to install and maintain in dual-helm boats.
In a hydraulic steering system, pressurized fluid is provided by a hydraulic pump attached to and rotated by the helm wheel used to steer the vessel. In other words, the human "helmsperson" is the pump "prime mover" and pumps of this type are known as helm pumps.
In a typical helm pump, there is an angled swash plate mounted stationary in the housing. The pump barrel has axial pistons reciprocating in piston bores in the barrel and each piston is in contact with the swash plate.
During the suction portion of a piston reciprocating "cycle," each piston is urged in one direction, usually rearward toward the helm wheel, by a spring and thereby draws fluid into the piston bore. During the pressure portion of the piston cycle, the piston is urged forward by its contact with the swash plate and delivers fluid to the steering circuit. For each revolution of the pump barrel (usually corresponding to a single revolution of the helm wheel), each piston makes one complete cycle comprised of one forward and one rearward motion component.
In a helm pump of the aforedescribed type, many design considerations involve the piston return springs. Such springs function in the following way.
Each piston is caused to move rearwardly by its spring and "draws" a slight vacuum in its bore so that fluid flows into such bore to be later pumped out. During this suction part of the cycle, it is important that each piston be kept in contact with the swash plate. A piston which cavitates significantly may separate from the swash plate and pump damage and/or improper steering may result.
To help overcome the tendency to cavitate during suction, earlier designers of helm pumps have used a relatively long, high-force spring to keep the piston in contact with the swash plate. The adverse implications for pump length (resulting from using a long spring) have been addressed by using a hollow piston. One end of the long piston spring is inserted into the piston interior cavity. Hollow pistons are more difficult to manufacture than solid pistons and give rise to manufacturing costs which, in view of the invention, are unnecessary.
And using a long spring and hollow piston increases the pump "unswept volume," i.e., the volume of fluid which is not expelled from the piston on each pumping cycle. Such unswept volume is that in the hollow piston and that between the piston end and the end of the barrel.
And, in turn, the use of a hollow piston gives rise to other design difficulties. When a hollow piston is used, the remaining wall thickness is insufficient to permit machining a seal groove and using a resilient seal in such groove and around the piston diameter. This problem could be overcome by making the piston diameter quite large but the resulting heavier, higher-drag piston may require an even higher-force spring.
Another approach (and the one commonly used) is to employ a metal-to-metal seal by "select-fitting" each piston to its bore. When select-fitting pistons to bores, the diameter of each bore is measured and pistons are segregated into groups, each according to a slightly different range of piston diameters. Then a piston is selected from a particular group to fit into a particular bore. Such selection is made so that the "clearance" between the piston and bore, i.e., the difference in diameter between a piston and its bore, is sufficiently small that leakage is maintained at an acceptably-low level. Select-fitting pistons to bores requires that both be precisely round within very close tolerances.
It is apparent that select-fitting is an expensive, time-consuming manufacturing process. And field repair is made much more expensive in that, in all likelihood, the barrel and pistons must be replaced as a set even though only the replacement of, say, a single piston is indicated.
Yet another disadvantage of prior art helm pumps involves system "bleeding." Bleeding is a procedure used to remove system air after initial installation or later service. Bleeding is necessary since air causes the system to feel "spongy" during steering. It is a known practice to return the often-air-laden fluid to the pump at a location where the pistons draw such air-laden fluid into the piston bore. With such arrangement, it is more difficult to remove all of the air from the system.
A new helm pump which addresses and overcomes some of the disadvantages of prior art helm pumps would be an important advance in the art.